If the earth rotated twice as fast as it currently does but its motion around the sun stayed the same would the length of the day change?

Answer 1

Interesting question. It depends on what time of day it was where you are when it reversed. If it was more than halfway through the light cycle of the day (sunup to sundown) then it would lengthen that perticular day assuming that the earth resumed the same speed of rotation that day but in the opposite direction. In other words if you have already gone through 8 hours of sunlight then you should get another 8 hours of sunlight for 16 total. If where you are has only gone through 3 hours of sunlight that morning then you will theoretically only get another 3 hours of sunlight as the sun was closer to the horizon and doesn't have that far to go before it sets again. Now here's the big clincher. If that happens then we all die and it won't really matter. The amount of gravity we have is dependant on the rotation of the earth. At the point of no rotation or even less rotation the gravity will change dramatically. God saw to it that everything on the earth was perfect for our survival. Speed of rotation, distance from the sun, temperature, atmosphere, etc... That is why there is no life (that we know of) on the other planets in our solar system.

The answer above by Freesinweasle is true, but I think the answer you are looking for is:

The question asks: How the lengeth of the solar day change?

Each time the Earth rotates around once on its axis, it moves a small distance along the its orbit around the Sun. Earth, therefore, has to rotate through slightly more than 360 degrees for the sun to return to the apparent location in the sky. Thus, the interval of time between noon one day and noon the next day(a solar day) is slightly grater than one true rotation period(one sidereal day). Our planet takes 365 days to orbit the Sun, so the additional angle is 360/365 = 0.986 degrees. Because Earth is rotating a the rate of 15 degrees per hour, takes about 3.9 minutes to rotate through this angel., the solar day is 3.9 minutes longer then the sidereal day(i.e, 1 sidereal day is roughly 23h 56m long). So the solar day would decrease by 7.8 minutes.

Answer 2

That's highly likely it would be a 12 hour day.